Mark Twain utilizes humor in several ways in his works.
Characters: Twain creates unforgettable and hilarious characters. Tom Sawyer is especially prominent as a clever and funny creation. His train of thought, crazy ideas, cruel pranks, childish logic, imagination, acting ability, and wit cause the reader to laugh aloud time and time again.
Dialogue: Mark Twain is a master of dialogue. Not only are Twain’s characters funny in and of themselves, but also the way they interact with each other causes smiles more often than not—their pointless arguments, clever insults, and plots against one another are often hilarious. As well as using dialogue, Twain utilizes the unique dialects and colloquialisms of the areas around the Mississippi River for his characters, not only giving them the authenticity only someone growing up in the region could, but also using that unique way of speaking to humorous advantage, twisting the accents, superstitions, and phrases into passages so packed with hilarity, the reader has to read them over again to experience the full effect.
Events: Twain often tosses his characters into ridiculous situations. For instance, after running away from home and realizing his family thinks him dead, Tom Sawyer finds he has the opportunity to attend his own funeral. Being the person he is, he takes this opportunity and runs with it, providing quite the entertainment for the reader. And this is only a small event in the life of Tom Sawyer. It seems that character can find himself in a funny pickle easier than not.
Words: Mark Twain describes funny characters, paints funny dialogues, and tells of funny events—and he does all this in a funny way with the words he chooses. Twain doesn’t just tell things blandly. He has a knack for writing in a witty and memorable fashion that leaves the reader laughing at more than just the elements of the story itself. It is the way his stories are told that sets him apart as a clever author.
If you want to laugh as well as think deeply about sensitive and important issues, Mark Twain is your man. His humor and way of depicting life in the 1800s are unforgettable and wonderfully funny.
Some simple examples of Twain's sense of humor can be found here.